1. Field of the Invention
The subject invention relates to ion mobility spectrometers, and particularly to the method of generating ions and the sampling of the ionic population at different intervals as the ion molecule reactions proceed to equilibrium.
2. Description of the Related Art
Ion mobility spectrometers have been used for many years to determine whether molecules of interest are present in a stream of gas. The prior art ion mobility spectrometers function by acquiring a sample that is to be tested for the presence of the molecules of interest. Some prior art ion mobility spectrometers acquire the sample by wiping a woven or non-woven fabric trap across a surface that is to be tested for molecules of interest. Other prior art ion mobility spectrometers create a stream of gas adjacent the surface to be tested for the molecules of interest or rely upon an existing stream of gas. The sample is transported on a stream of inert gas to an ionization chamber. The prior art ion mobility spectrometer exposes the sample to a radio active material in the ionization chamber. The radio active material, such as nickel63 or tritium bombards the sample stream with xcex2-particles and creates ions.
The prior art ion mobility spectrometer further includes a drift chamber in proximity to the ionization chamber. The drift chamber is characterized by a plurality of field-defining electrodes and a collector electrode at the end of the drift chamber opposite the ionization chamber. Ions created in the ionization chamber are permitted to drift through the drift chamber and toward the collector electrode. The collector electrode detects and analyzes the spectra of the collected ions and provides an appropriate indication if molecules of interest are detected.
Ion mobility spectrometers have many applications, including security applications where the ion mobility spectrometer is used to search for and identify explosives, narcotics and other contraband. Examples of ion mobility spectrometers are shown in U.S. Pat. No. 3,699,333 and U.S. Pat. No. 5,027,643.
Improvements to the above-described early ion mobility spectrometer have been developed by Ion Track Instruments, Inc. and are referred to as ion trap mobility spectrometers. The ion trap mobility spectrometer provides greater sensitivity and reliability over the above-described ion mobility spectrometer. An example of an ion trap mobility spectrometer is described in U.S. Pat. No. 5,200,614 which issued to Anthony Jenkins. This prior art ion trap mobility spectrometer achieves improved operation by increasing ionization efficiency in the reactor and ion transport efficiency from the reactor to the collector electrode. More particularly, the ionization chamber of the ion trap mobility spectrometer is a field-free region where the ion population of both electrons and positive ions is allowed to build up by the action of the xcex2-particles on the carrier gas. The high density of ions produces a very high probability of ionization of the molecules of interest, and hence an extremely high ionization efficiency.
U.S. Pat. No. 5,491,337 shows still further improvements to ion trap mobility spectrometers. More particularly, U.S. Pat. No. 5,491,337 discloses an ion trap mobility spectrometer with enhanced efficiency to detect the presence of alkaloids, such as narcotics.
Despite the operational efficiencies described in the above-referenced patents, there is a demand for still further improvements that enable cost reductions while increasing the resolution or selectivity of the spectrometer. There are also regulatory barriers to using radioactive material in some countries which prevents the use of portable applications of equipment containing a radioactive source.
Recent attempts to provide an electronic means of ionization have been described in U.K. Patent Appl. No. 98164452. This does not however provide for ionic reactions to occur in zero field conditions or to probe these reactions as they proceed to equilibrium. Subsequently the method is both less sensitive and less selective than that described herein.
The subject invention is directed to an ion trap mobility spectrometer that replaces the radioactive ionization source with a source of ions produced by high voltage electronic pulses. Ions are formed periodically in a reaction chamber and are allowed to maximize their population and thermalize in a field-free environment and then react with molecular species in the gas phase in the reaction chamber. After a short time, the ions are pulsed into the drift section of an ion trap mobility spectrometer, such as the drift section of the ion trap mobility spectrometer disclosed in U.S. Pat. No. 5,200,614. The reaction period may be varied to sample the ion population at different intervals. This enables the ion-molecule reactions to be monitored as the ion population approaches equilibrium. Results then can be analyzed to determine differences between reacting species because the molecular ion population varies at different time points approaching equilibrium. Thus, there is an improved identification of targets.